1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly to a plasma display panel that is adapted for reducing load effect and improving luminescence efficiency and discharge efficiency, and a driving method thereof.
2. Description of the Related Art
Recently, various flat panel displays, which generally weigh less and are smaller in size than might reduce cathode ray tubes, have been developed. These flat panel displays include liquid crystal displays (hereinafter ‘LCD’), field emission displays (hereinafter ‘FED’), plasma display panels (hereinafter ‘PDP’) and electro-luminescence EL displays.
The PDP, in particular, uses a gas discharge and it has the advantage that it can be easily produced in a large sized panel. FIG. 1 shows a common three electrode AC surface discharge PDP, which employs three electrodes and is driven by AC voltage.
Referring to FIG. 1, the discharge cell of the three electrode AC surface discharge PDP includes a scan electrode Y and a sustain electrode Z formed on an upper substrate 10, an address electrode X formed on a lower substrate 18. Both the scan electrode Y and the sustain electrode Z include a transparent electrode 12Y, 12Z and a metal bus electrode 13Y, 13Z, where the metal bus electrode has a narrower width than the transparent electrode. Further, the metal bus electrode is formed at one side edge of the transparent electrode as shown.
The transparent electrodes 12Y, 12Z are formed of indium tin oxide (ITO) on the upper substrate 10 in the related art. The metal bus electrodes 13Y, 13Z are formed of a metal, such as chrome (Cr), on the transparent electrodes 12Y, 12Z and they reduce the voltage drop which is caused by the high resistivity of the transparent electrodes 12Y, 12Z. A dielectric layer 14 and a passivation film 16 are deposited on the upper substrate 10 where the scan electrode Y and the sustain electrode Z are formed in parallel. A wall charge generated, as a result of a plasma discharge, is accumulated in the upper dielectric layer 14. The passivation film 16 prevents the loss of the upper dielectric layer 14 caused by the sputtering associated with the plasma discharge. This increases the emission efficiency of secondary electrons. The passivation film 16 is made of magnesium oxide MgO in the related art.
A lower dielectric layer 22 is formed on the lower substrate 18 where the address electrode X is formed, and a phosphorus layer 26 is spread over the surface of barrier ribs 24 and the lower dielectric layer 22. The address electrode X is formed in a direction that crosses the scan electrode Y and the sustain electrode Z (i.e., a perpendicular direction). The barrier ribs 24 are formed parallel to the address electrode X to prevent ultraviolet and visible light, which are generated by the discharge, from leaking into adjacent discharge cells. The phosphorus layer 26 is excited by the ultraviolet light, which is generated upon the plasma discharge, to generate any one of red, green and blue visible light, depending on the type of phosphor coating that discharge cell. An inert gas mixture is injected into a discharge space between the upper/lower substrate 10, 18 and the barrier ribs 24.
Each display time frame for the three electrode AC surface discharge PDP is divided into a plurality of subfields, wherein the light emission associated with each subfield differs proportionally, thereby achieving various gray levels for displaying an image. Each subfield is further divided into a reset period, an address period, a sustain period and an erasure period.
Herein, the reset period is a period during which uniform wall charges are formed in the discharge cell. The address period is a period during which a selective address discharge is generated in accordance with the logical value of the video data, thus selecting or not selecting each discharge cell for illumination during that subfield. The sustain period is a period during which a discharge is maintained in those discharge cells that were selected during the address period. The erasure period is a period during which the sustain discharge generated during the sustain period is eliminated.
In AC surface discharge PDPs that are driven as described above, a high voltage of not less than several hundred volts is required to achieve the address discharge and the sustain discharge. Accordingly, an energy recovery unit is used for minimizing the power required to achieve the address discharge and the sustain discharge. The energy recovery unit recovers the voltage between the scan electrode 12Y and the sustain electrode 12Z, and utilizes the recovered voltage as a driving voltage for the next discharge.
FIG. 2 depicts an energy recovery unit 30, 32 for a PDP as proposed in U.S. Pat. No. 5,081,400. As shown, the energy recovery units 30, 32 are symmetrically installed with a capacitive load Cp, i.e., a panel capacitor therebetween. The panel capacitor Cp equivalently represents the capacitance which is formed between the scan electrode Y and the sustain electrode Z. The first energy recovery unit 30 supplies a sustain voltage to the scan electrode Y and the second energy recovery unit 32 supplies the sustain voltage to the sustain electrode Z. The first energy recovery unit 30 and the second energy recovery unit 32 alternate in operation with respect to each other.
The components of the energy recovery units 30, 32 of the related art PDP are now described with reference to the first energy recovery unit 30. Otherwise, the first and second energy recovery units 30, 32 are the same. The first energy recovery unit 30 includes an inductor L connected between the panel capacitor Cp and a source capacitor Cs; first and third switches S1, S3 connected in parallel between the source capacitor Cs and the inductor L; a second switch S2 connected between a sustain voltage source Vs and a first node N1 between the panel capacitor Cp and the inductor L; and a fourth switch S4 connected between the first node N1 and a ground voltage source GND.
The source capacitor Cs recovers the voltage stored in the panel capacitor Cp during a sustain discharge, and it re-supplies voltage to the panel capacitor Cp. The voltage of Vs/2 corresponding to half the value of the sustain voltage Vs charges the source capacitor Cs. The inductor L forms a resonance circuit together with the panel capacitor Cp. To achieve this, the first through the fourth switches S1 to S4 control the flow of electric current. On the other hand, the fifth and sixth diodes D5, D6 each installed between the first and second switches S1, S2 and the inductor L prevent the current from flowing in a reverse direction.
FIG. 3 is a timing and waveform diagram representing the output waveform of the panel capacitor Cp and the corresponding switching states of the switches S1 through S4 of the first energy recovery unit 30.
Before period T1, it is assumed that the panel capacitor Cp has a charge of 0 volts, and the source capacitor Cs has a charge of Vs/2 volts. The operation of the first energy recovery unit 30 is now described in detail.
During the period T1, the first switch S1 is turned on to form a current path from the source capacitor Cs to the panel capacitor Cp through the first switch S1 and the inductor L. Accordingly, the voltage Vs/2 stored in the source capacitor Cs is supplied to the panel capacitor Cp. At this moment, the inductor L and the panel capacitor Cp form a series resonance circuit, thus the sustain voltage Vs, which is double the voltage Vs/2 of the source capacitor Cs, charges the panel capacitor Cp.
During the period T2, the first switch S1 remains in an on-state and the second switch S2 is turned on. When the second switch S2 is turned on, the sustain voltage Vs from the sustain voltage source is supplied to the scan electrode Y. The sustain voltage Vs supplied to the scan electrode Y prevents the voltage of the panel capacitor Cp from dropping below the sustain voltage Vs, thus causing the sustain discharge to be generated in a normal manner. Because the panel capacitor Cp is charged to the sustain voltage Vs during the period T1, the amount of drive power supplied from the outside needed to generate the sustain discharge is minimized.
At the beginning of the period T3, the first switch S1 is turned off. During the period T3, the scan electrode Y remains at the sustain voltage Vs.
At the beginning of the period T4, the second switch S2 is turned off and the third switch is turned on. When the third switch S3 is turned on, a current path forms from the panel capacitor Cp to the source capacitor Cs through the inductor L and the third switch to recover the voltage stored in the panel capacitor Cp. At this moment, the source capacitor Cs charges to the voltage of Vs/2.
At the beginning of the period T5, the third switch S3 is turned off and the fourth switch S4 is turned on. When the fourth switch S4 is turned on, a current path forms between the panel capacitor Cp and the ground voltage source GND, thus the voltage of the panel capacitor Cp drops to 0V.
During the period T6, the state of the switches S1 through S4 and the 0V stored at the panel capacitor Cp are maintained. An AC drive pulse supplied to the scan electrode Y is achieved by repeating the aforementioned switching sequence at a predefined interval.
On the other hand, the second energy recovery unit 32 alternately supplies the drive voltage to the panel capacitor Cp. Accordingly, the panel capacitor Cp receives a sustain voltage Vs that has a different polarity. In this way, the sustain voltage Vs having the different polarity is supplied to the panel capacitor Cp, thus the sustain discharge is generated at the discharge cells.
However, the discharge efficiency, the luminescence efficiency and the power consumption associated with the related art energy recovery units varies in accordance with the load effect of the PDP. This is a problem because the picture quality of the PDP then changes in accordance with the load of the PDP. For instance, if the PDP load is small, one discharge is generated for each sustain pulse; but, on the other hand, if the PDP load is relatively big, two discharges may occur for one sustain pulse. Accordingly, a PDP design and method are needed that increase PDP display quality regardless of load.